1. Field of the Invention
This invention relates generally to glow plugs of the type for assisting cold start combustion in a combustion chamber, and more particularly toward a glow plug having a metallic heater probe.
2. Related Art
Glow plugs are typically used in applications where a source of intense heat is required to either directly initiate or to aid in the initiation of combustion. As such, glow plugs are used in space heaters, industrial furnaces and diesel engines to name a few. Glow plugs used in diesel engine applications are usually categorized as either open coil type or sheathed type devices. Sheath type glow plugs are then divided between ceramic type heater probes and metallic type heater probes. In a metallic type sheath heater probe, one or more spiral wound resistive wires are contained within a metallic sheath, embedded in an electrically insulating and thermally conductive powder. A glow plug of this type is described, for example, in U.S. Pat. No. 4,963,717. The electrical resistance wire(s) located in the sheath are totally embedded in the insulating powder and the insulating powder is sealed in the sheath using an elastomeric o-ring seal or other gasketing device.
Metallic type sheathed heater probes are normally inserted into the glow plug shell by mechanical interference fit. An interference fit requires a high strength from both the probe and the shell, together with accurate manufacturing tolerances. The requirement for high strength limits the minimum metal thicknesses which can be used in these applications, leading to a minimum possible diameter at the shell-tube-probe joint. This requirement similarly leads to a minimum possible diameter for the probe, which is currently around 4 millimeters. Thus, the joint surface (probe-to-shell) must have at least this diameter using present techniques.
Management of a diesel engine may be improved if combustion chamber pressures are monitored in real time. Pressure sensors can be introduced as stand-alone devices, or more preferably as integrated into a glow plug. One design of integrated glow plug pressure sensor uses a flexible membrane provided between the heater probe and shell. This increases the glow plug dimensions and further deters miniaturization of the various glow plug components. According to current techniques, the use of a metallic probe currently limits the minimum diameter of glow plug designs of this type, because there is not enough space for the membrane and the membrane is not strong enough to support an interference fit with the probe. Therefore, using current techniques, ceramic probes are typically used in this type of integrated pressure sensor applications to achieve a small glow plug diameter. When ceramic probes are used, the diameters can be reduced to about 3.2 millimeters using current technology, which diameter reduction allows the entire glow plug diameter to be similarly reduced. However, because ceramic probes are more expensive than metallic heater probes, an increase in glow plug cost results.
Accordingly, there is a desire to use small diameter metallic heater probes in glow plug applications so as to attain a large cost saving.